Polybutylene terephthalate resin has good mechanical and electrical properties and chemical resistance. In particular, it has a rapid crystallization rate, which gives the polybutylene terephthalate excellent molding characteristics. For this reason, polybutylene terephthalate is drawing attention as a thermosetting resin for injection molding and as a substitute for metal, and it is used in automotive, electric, and electronic industries. However, since polybutylene terephthalate resin has a glass transition temperature of about 40 to 60° C., it has a low thermal distortion temperature. Also, since polybutylene terephthalate has low impact resistance at room temperature as well as a low thermal distortion temperature, research has focused on the development of a polyester/polycarbonate alloy for high impact resistance applications.
U.S. Pat. Nos. 4,393,153, 4,180,494, 4,906,202, and 4,034,013 disclose a method of improving impact resistance by adding ethylene-propylene copolymer (EPR), ethylene-propylene-diene copolymer (EPDM), or a methylmethacrylate-butadiene-styrene copolymer (MBS) into a polyester resin. However, when these additives are added without a compatibilizer, the phases of polyester and polycarbonate become unstable, which may lead to deterioration of mechanical properties.
European Patent Nos. 33,393 and 180,417 disclose a method of increasing compatibility by introducing a functional group into a polyester resin, or a method of cross-linking a terminal group of a polyester resin. The above methods may improve the impact resistance of a polyester resin but they leave un-reacted functional groups, which can cause a color change and generate gas during injection molding. In particular, when glycidyl methacrylate is used, the external surface of an injection molded product can turn milky-white so that the product typically has to be painted before use.